Different electron and phonon populations can be driven out of local thermal equilibrium in nanoelectronic devices, laser materials processing, and thermal transport measurements. A better understanding of the highly non-equilibrium transport phenomena is necessary for the design of next-generation devices and material structures with enhanced performance and reliability. However, current experimental capabilities are inadequate for probing local temperatures of these different energy excitations. Despite the recent progresses in scanning thermal microscopy (SThM), infrared spectroscopy, and micro-Raman spectroscopy, there is a lack of experimental methods for resolving the local temperature of the acoustic phonons that dominate heat conduction, as well as low-frequency acoustic phonons that may be in the ballistic transport regime in nanostructures. The objective of this research is to investigate new experimental methods for probing the local temperature of acoustic phonons during highly non-equilibrium transport processes in nanostructures and devices. The techniques to be investigated include a new method based on micro-Brillouin light scattering (BLS) for probing the local temperature of low-frequency (0.5 GHz to 100?s GHz) acoustic phonons with sub-micron spatial resolution. Based on preliminary BLS measurements of local acoustic phonon temperatures in glass, this technique will be investigated further for probing the local acoustic phonon temperature in silicon nanostructures that are either electrically biased or optically excited. The obtained acoustic phonon temperature will be correlated with those measured by micro-Raman spectroscopy, infrared spectroscopy, and SThM to quantify local non-equilibrium between electrons, acoustic and optical phonons.
The demonstration of the micro-BLS technique as a thermal microscopy tool for low-frequency phonons will be of value for the experimental thermal transport research community. Meanwhile, the measurement data can be used by theoretical and computational thermal transport researchers to establish a better understanding of several intriguing and important non-equilibrium transport phenomena. Such understanding can impact further advances in nanoelectronic devices, laser materials processing, and thermal measurement techniques. In addition, this research will provide student training opportunities in state-of-the-art experimental techniques, and result in new example materials for undergraduate and graduate courses. It will also generate new demonstration materials to be used in outreach activities for attracting students from underrepresented groups to engineering and science professions, and for exposing university research to K-12 students, parents, and teachers in Texas.